A variety of rheometers are known for measuring rheological properties such as viscosity and elasticity of sample liquids. For example, US 2008/0134765 A1 discloses a micro-rheometer having micro-fabricated flow channels incorporating geometry changes to form constriction regions. The disclosed micro-rheometer also comprises an array of pressure sensors embedded within material forming one side of the flow channel. In this way, the pressure sensors are not in direct contact with fluid flowing within the flow channel. The thickness of material separating the pressure sensors from the flowing liquid is arranged such that the pressure sensors are able to provide measurements indicative of the pressure of the flowing fluid at the various locations of the sensors. The motivation for embedding (i.e. integrating) the pressure sensors in the body of material defining the flow channel in this way is to ensure that the flow of fluid within the channel is not disturbed or perturbed by the pressure sensors; a smooth, uninterrupted internal surface of the channel is provided. Although such apparatus is of use, it will be appreciated that disadvantages associated with the integrated pressure sensor arrangement are that the apparatus may be difficult and/or expensive to produce, once the arrangement has been manufactured there is no flexibility in where to locate the pressure sensors, and the separation of the pressure sensors from the flowing liquid by even a small thickness of material can reduce the accuracy of the pressure measurements obtained on the sample. Moreover, it only measures steady state viscosity of sample liquids.